It’s a surprise move from the Ohio Republican, who served as the chief trade negotiator for President George W. Bush and who supported the North American Free Trade Agreement and a series of subsequent trade deals.

Portman defended his position on Thursday as consistent, saying he has always supported free trade as long as it’s also fair to American workers.

“I can’t support this (Trans-Pacific Partnership) in its current form because it doesn’t provide that level playing field,” Portman said in a call with Ohio reporters Thursday.

But Portman’s Democratic foes called the senator’s announcement an election-year conversion on a hot-button economic issue.

"For decades, Senator Rob Portman has turned his back on Ohio's workers every chance he got — prioritizing China’s interests at the expense of our working families,” said former Gov. Ted Strickland, a Democratic candidate in the Senate race. He noted that Portman supported a fast-track bill that allows the trade pact to be "rammed" through Congress, only to turn around and say he'd vote against the deal.

The Trans-Pacific Partnership is the largest trade agreement since NAFTA. Supporters say it will knock down barriers for U.S. exports, create American jobs, and make the U.S. more competitive in the global marketplace.

But opponents, including Sen. Sherrod Brown, D-Ohio, say the trade deal will cost jobs and depress wages, as more American companies shift jobs overseas where labor and environmental laws are more lax. Union leaders have called the deal a giveaway to multinational corporations.

As Bush’s trade representative, Portman helped negotiate such free trade agreements, and he notched a big win in 2005 when Congress passed the Central American Free Trade Agreement, a priority for the Bush administration.

"From an economic point of view, trade will always be something positive for growth," Portman said in a 2005 interview.

Last year, as the Obama administration was negotiating the TPP, Portman voted in favor of fast-track legislation, which allows the president to send the final trade deal to Congress for a vote, with no opportunity for amendments.

Portman said Thursday he has several concerns about the final TPP agreement, which global negotiators signed on Thursday morning in New Zealand. He said the deal doesn’t do enough to crack down on currency manipulation, a practice in which countries — most notably China — devalue their currency to make their products cheaper in the global marketplace. Portman has fought for years to curb currency manipulation, which puts U.S. manufacturers at a disadvantage.

Portman said he also objected to the “rules of origin” for automobiles included in the final TPP. Under the agreement, only 45 percent of a vehicle would have to be made in a TPP country to qualify for the agreement’s benefits — including tariff-free importation to the American market. Brown has made the rules of origin a chief argument in his efforts to torpedo the deal, saying it could allow a car to be made mostly from Chinese parts, then assembled in Japan, and sold in the U.S. tariff-free.

Asked on Thursday if he has ever opposed a trade agreement, Portman did not directly answer the question.

“I have always made my views very well known on trade,” he said. “I’m for expanding exports, opening up markets … but I’ve also always said it’s got to be fair.”

His decision to oppose the TPP, he said, is “consistent with what I have said from the start on this.”

Two of his rivals in the 2016 Senate race disagreed.

"When it comes to women, Rob Portman is anti-choice. On trade, he's multiple choice," said Dale Butland, a spokesman for P.G. Sittenfeld, a Democratic city councilman from Cincinnati vying to oust Portman in the 2016 Senate race. “That's exactly why voters don't trust Washington or career politicians who will say anything to get elected.”

Both Sittenfeld and Strickland came out strongly against the TPP last year.

The trade issue could play an outsized role in the Senate race. That’s because manufacturing is one of the largest sectors of Ohio’s economy, providing more than 660,000 jobs. And the Buckeye State is the ninth-largest exporter in the nation, with $52 billion of commodities shipped to other countries in 2014.

Jennifer Duffy, who analyzes Senate races for the nonpartisan Cook Political Report, said coming out in favor of the TPP could have hurt Portman with Ohio voters who are already anxious about the economy and their job security.

“Voters have become very skeptical of (free trade agreements),” Duffy said. That is particularly true, she said, in states like Ohio that have seen their manufacturing bases erode in a global economy.

Duffy said Portman’s decision to oppose the TPP could be seen as a political decision aimed at bolstering his re-election prospects. But because of his experience as a trade negotiator, Portman is more credible on the issue than other candidates, Duffy said.

“He knows how (such deals) should be written … and I think that he can speak credibly about why he’s opposing it,” she said.